B6 - Inheritance, Variation, and Evolution

• Sexual Reproduction

  Sexual reproduction involves the joining (fusion) of male and female gametes:In sexual reproduction there is mixing of genetic information which leads to variety in the offspring. The formation of gametes involves meiosis, which leads to non-identical cells being formed.

  • sperm and egg cells in animals
  • pollen and egg cells in lowering plants.
• Asexual Reproduction

  Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved.
• Meiosis

  Cells in reproductive organs divide by meiosis to form gametes.When a cell divides to form gametes:

  
  

  • copies of the genetic information are made
  • the cell divides twice to form four gametes, each with a single set of chromosomes
  • all gametes are genetically different from each other.

  Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.
• Advantages of sexual reproduction

  • produces variation in the offspring
  • if the environment changes variation gives a survival advantage by natural selection
  • natural selection can be speeded up by humans in selective breeding to increase food production.
• Advantages of asexual reproduction

  • only one parent needed
  • more time and energy efficient as do not need to find a mate
  • faster than sexual reproduction
  • many identical offspring can be produced when conditions are favourable.
• What organisms reproduce by both asexual and sexual reproduction?

  • Malarial parasites reproduce asexually in the human host, but sexually in the mosquito.
  • Many fungi reproduce asexually by spores but also reproduce sexually to give variation.
  • Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils.
• DNA

  • The genetic material in the nucleus of a cell is composed of a chemical called DNA. DNA is a polymer made up of two strands forming a double helix. The DNA is contained in structures called chromosomes.
  • A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
  • The genome of an organism is the entire genetic material of that organism. The whole human genome has now been studied and this will have great importance for medicine in the future.
• The Human Genome

  The genome of an organism is the entire genetic material of that organism. The whole human genome has been studied, and this has great importance for medicine.

  In order to exploit its secrets, it is vital that the human genome is fully understood.

  It enables us to:

  • search for genes linked to different types of disease
  • understand inherited disorders and their treatment
  • trace human migration patterns from the past
• Gene Diseases

  Scientists are studying genes associated with breast cancer, specifically BRCA1 and BRCA2.

  • Mutations in these genes account for about 10% of all inherited breast cancer cases.
  • Pedigree analysis was used to identify these genes, illustrating the inheritance pattern of the disease.
  • DNA testing of affected and unaffected individuals was conducted to identify differences.
  • The presence of these genes can now be detected through a simple blood test.

  
• DNA Structure

  • DNA is a polymer made from four different nucleotides.
  • Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar.
  • DNA contains four bases, Adenine, Cytosine, Guanine and Thymine.
  • A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.
  • DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases.
  • The DNA polymer is made up of repeating nucleotide units.

  
• Protein Synthesis

  • DNA code for protein remains in the nucleus, while mRNA copy moves to ribosomes for synthesis in the cytoplasm.
  • Protein produced depends on template used, and changes can result in different proteins.
  • Carrier molecules add specific amino acids to the growing protein in the correct order.
  • DNA structure determines protein synthesis, and changes can lead to different proteins.
  • mRNA copy moves to the ribosome, where amino acids are connected in a specific order to create a specific protein molecule.

  
• Mutations in protein synthesis

  • Mutations occur continuously. Most do not alter the protein, or only alter it slightly so that its appearance or function is not changed.
  • A few mutations code for an altered protein with a different shape. An enzyme may no longer fit the substrate binding site or a structural protein may lose its strength.
  • Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off, so variations in these areas of DNA may affect how genes are expressed.

  
• What is a gamete?

  A gamete is a reproductive cell that carries half the number of chromosomes as a regular body cell, and is involved in sexual reproduction.
• What is a chromosome?

  A thread-like structure made up of DNA
• What is a gene?

  A gene is a segment of DNA that contains the instructions for building a specific protein or performing a specific function in an organism.
• What is an allele?

  An allele is a variant form of a gene. It is one of the possible versions of a gene that can exist at a specific location on a chromosome. Alleles can differ in their DNA sequence, resulting in different traits or characteristics.
• What is a dominant allele?

  An allele that is expressed in the phenotype when present in the genotype.
• What is a recessive allele?

  An allele that is only expressed when paired with another recessive allele.
• What does it mean when a allele is homozygous?

  Two identical alleles for a particular gene.
• What does it mean when an allele is heterozygous?

  Two different alleles for a gene.
• What is a genotype?


  The chemical composition of an organism's DNA which gives rise to its observable traits (Phenotype).
• Inherited disorders

  Some disorders are inherited. These disorders are caused by the inheritance of certain alleles.

  • Polydactyly (having extra fingers or toes) is caused by a dominant allele.
  • Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
• Genetic tests

  • Analyses DNA to identify alleles causing genetic disorders.
  • Can be conducted at any stage of life.
  • Antenatal testing analyses DNA or chromosomes before birth.
  • New born blood spot test detects genetic disorders early.
  • Pre-implantation genetic diagnosis (PGD) uses fertility drugs to stimulate egg release.
  • In vitro fertilisation (IVF) is used to collect and fertilize eggs.
  • After eight-cell stage, one cell is removed and tested for disorder-causing alleles.
  • Embryos without disorder allele are implanted into the uterus.
• Sex determination

  Ordinary human body cells contain 23 pairs of chromosomes.22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.

  • In females the sex chromosomes are the same (XX).
  • In males the chromosomes are different (XY).
• Variation

  Differences in the characteristics of individuals in a population is called variation and may be due to differences in:

  • the genes they have inherited (genetic causes)
  • the conditions in which they have developed (environmental causes)
  • a combination of genes and the environment.

  There is usually extensive genetic variation within a population of species. These variants all arise from mutations and most have no effect on the phenotype.
• Mutations

  Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species.
• Define Evolution

  A change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species.
• Theory of evolution

  • The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
  • Evolution occurs through natural selection of variants that give rise to phenotypes best suited to their environment.
  • If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring they have formed two new species.
• Define selective breeding

  Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics. Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.
• Selective breeding

  Selective breeding involves choosing parents with the desired characteristic from a mixed population. They are bred together. From the offspring those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.
• Characteristics for selective breeding

  The characteristic can be chosen for usefulness or appearance:

  • Disease resistance in food crops.
  • Animals which produce more meat or milk.
  • Domestic dogs with a gentle nature.
  • Large or unusual lowers.

  Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.
• Define Genetic Engineering

  A process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.
• How have different organisms been genetically engineered?

  • Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits.
  • Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.
• Genetic engineering - pros and cons

  • Genes from human and other organisms can be 'cut out' and transferred to other cells.
  • Genetically modified (GM) crops are resistant to insect attack or herbicides.
  • GM crops generally show increased yields.
  • Concerns include effects on wild flowers and insects.
  • Some believe GM crops' health effects are underexplored.
  • Modern medical research explores GM to overcome inherited disorders.
• Process of genetic engineering

  • enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus
  • the vector is used to insert the gene into the required cells
  • genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics.
• Cloning - Tissue Culture


  Using small groups of cells from part of a plant to grow identical new plants. This is important for preserving rare plant species or commercially in nurseries.
• Cloning - Cuttings


  An older, but simple, method used by gardeners to produce many identical new plants from a parent plant.
• Cloning - Embryo Transplants


  Splitting apart cells from a developing animal embryo before they become specialised, then implanting the identical embryos into host mothers.
• Cloning - Adult cell cloning


  • The nucleus is removed from an unfertilised egg cell.
  • The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell.
  • An electric shock stimulates the egg cell to divide to form an embryo.
  • These embryo cells contain the same genetic information as the adult skin cell.
  • When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.
• Theory of evolution

  Charles Darwin, as a result of observations on a round the world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection.

  • Individual organisms within a particular species show a wide range of variation for a characteristic.
  • Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.
  • The characteristics that have enabled these individuals to survive are then passed on to the next generation.